I think this is a great idea. I actually had a similar idea and am starting a project, but with a couple differences...
- no configs - Mine will have a config that is can be overridden in game or that can contain Mod preset quests that can be disabled in config or in game.
- all in game config - See above
- no commands - Mine will include the option of doing things via command since it is sometimes faster.
- Op pulls in block, possible 2 blocks high like a refrigerator - I am going to have a special item for this instead of a block however you can set a sign with a quest by hitting it with the special item.
- right clicks block and gets a gui - See above
- gui has 2 slots - Optional slots for extended possibilities with quests
- slot one is titled condition, and a ghosted quantity of items is put there
- slot two is titled rewards, where another ghosted quantity of items is located
- op then puts a sign or more on the machine sides or near (not required by the mod) explaining the quest
- player right clicks machine, end is instructed to fill in condition requirement until the reward becomes unghosted - Give the option to have an NPC give the quest (think like villagers but a bit more info for players than is provided by the villager gui)
- at which time they take the reward and the condition self nukes - If this is a NPC than they would just give the item and leave/have a new quest/ or the sign assigned the quest is destroyed
- completion is stored in the scoreboard
- there could probably be an option where if op places an repeater in a slot, that it becomes a repeatable quest - Also the option to do this through commands since my mod will have some option commands for ease of use.
My mod will be available for 1.9.4+ once it is done.